1. Field of the Invention
The present invention relates to an automatic control system for a unit operation (e.g. a distillation tower or anode adjustment system of a chlor/alkali cell). In particular, the present invention relates to an automatic control system for a unit operation that involves measuring at least one process parameter in a real unit operation and both estimating the same parameter or parameters in a real time dynamic simulation of the unit operation.
2. Brief Description of Art
In 1993, it was has estimated that there are about 40,000 distillation columns in operation in the U.S. chemical processing industries (CPI). These columns are estimated to consume 3% of the total U.S. energy usage. Moreover, distillation columns comprise 95% of the separation processes for the CPI. Since distillation control directly affects product quality, production rates, and utility usage, distillation control has great economic importance.
In operation of continuous distillation, the chemical feed is added to a distillation column anywhere from the bottom to the top of the column, depending on the intended purpose. Heat added to a reboiler at the bottom of the column vaporizes a substantial portion of the feed, and cooling added to a condenser at the top of the column condenses at least a portion of the vapor from the top of the column. The section of the column above the feed point and below the condenser is referred to as the rectification section, and the section between the feed point and the reboiler is known as the stripping section. In the operation of a column the ratio of the flow of liquid to vapor and the relative volatility of the components determines how composition changes from stage to stage. Control over the performance of a distillation column is further accomplished by changing the heat added to the reboiler to change the vapor flow (boil up) and by controlling the amount of condensate that is recycled back to the top of the column (reflux) flow.
In general, changes in the reflux flow or the boil up rate individually alters the ratio of distillate to bottoms product flows and thus their composition. However, increasing both the boil up rate and the reflux flow so that the distillate and bottoms flows remain constant tends to increase the separation ability of the column. Logically, decreases of these flows have the opposite effect.
There are several difficult problems to be overcome in the control of a distillation column.
(a) Inability to tightly control the composition of both distillate and bottoms products simultaneously.
(b) Small changes in feed composition result in large relative changes in product purity.
(c) Column upsets require several hours to correct.
(d) High purity distillation control is difficult to accomplish without the use of on-line analyzers.
(e) Distillation requires much manual control during startup.
(f) Distillation consumes more energy than necessary and is less productive when reflux ratio is set at a fixed (too high) value, but this is a common approach to control.
These problems are described in the following general operating situations: first, there is a substantial quantity of liquid and vapor contained in each tray of the column during operation. Changes of feed composition, boil up, or reflux flow introduce disturbances in the column that require a substantial length of time before product compositions are affected, and substantially longer before they reach steady state following a disturbance. Once a distillation column is operating outside its specifications, it may take hours to bring it back to the desired operating conditions. One advantage of the present invention is that it provides a means to measure and respond to disturbances inside the distillation column before these disturbances affect composition of either distillate or bottoms product. The present invention improves the overall stability of column operation.
Another problem is that the steady state impact of any of these disturbances is highly non-linear. For example, if a small decrease in boil up rate increases the quantity of light component or components in the bottoms product from 1% to 2%, the same small increase in boil up rate may decrease them to only about 1/2%. This non-linearity causes conventional control systems to become unstable when they operate at conditions of lower purity than where they were tuned for optimum control. A second advantage of the present invention is that it provides a measure of the state of the column that is linear over the entire normal operating range. This enables startup of a column under automatic control. It also provides a linearizing method for composition control of high purity products.
Another difficulty often encountered is that the dynamic behavior of the column is highly complex. The initial response of distillate or bottoms composition to a change in an input variable is often in the opposite direction to that of the long-term steady state effect. Also, the time required between a change in input and approach to a new steady-state condition strongly depends on the amount and direction of the change. It is an advantage of the present invention that this complex dynamic behavior can be accurately predicted using a real-time dynamic simulation.
Disturbance rejection is a desirable property of the control of a distillation column. An efficient control algorithm should be able to prevent an unmeasured change in feed composition from affecting either distillate or bottoms composition significantly. The ratio of the relative change of product composition to the change in feed composition is the disturbance rejection ratio, (the smaller the better). Another advantage of the present invention is that it can decrease the disturbance rejection ratio below that achieved by other methods of the prior art applied to the same problem.
A particular problem for columns that operate with highly pure products at both distillate and bottoms is that a small change in feed composition may lead to a much larger relative change in composition of one or the other product. For this type of system, the disturbance rejection ratio of the system may be larger than 100% even while under sophisticated control strategies of the prior art. By employing the present invention, a disturbance rejection ratio less than 100% can be achieved.
Yet another difficulty with controlling high-purity distillation is that product compositions are not easy to measure. The boiling point of a high purity product changes very little with its purity, so an on-line chromatograph is often employed for this purpose. Such on-line analytical devices obtain adequate accuracy only when there is sufficient time required to separate the components inside the chromatograph. This time adds dead time to feed back control and makes it less able to reject disturbances. In addition, reliable performance of delicate analytical equipment is difficult to guarantee in a plant environment. Yet another advantage of this invention is that it allows precise control of the composition of products of a distillation even in the absence of on-line analyzers. The real-time dynamic simulation of this method provides an instantaneous "software analyzer," whose signals may be used for feedback control to achieve better disturbance rejection than for equivalent control strategies using a real analyzer.
Yet another difficulty of control is that each distillation column has a maximum capacity for liquid and vapor flows. When this limit is exceeded in any part of the column, liquid accumulates in the column. This behavior is known as flooding. Control systems must be designed to limit boil up and reflux rates in order to avoid flooding, but the maximum allowable vapor flow and liquid flows are also interdependent. As the boil up rate approaches flooding conditions, the vapor flow entrains some liquid from the lower stage. This entrapment decreases the apparently separation efficiency of the column. The control methods of this invention incorporate limits that will prevent flooding, and the dynamic simulation aspect of the present invention incorporate a method of continuously estimating the separation efficiency.
Yet another problem with distillation control is that another disturbance introduced into a distillation column is a change in the internal reflux rate that is caused by a change in reflux temperature coming from the condenser. Particularly for air cooled condensers, rain showers can cause rapid decreases in reflux temperature and thus significantly upset a distillation column. This invention addresses this method by incorporating reflux temperature into the dynamic simulation.
To deal with these difficulties, some distillation operations rely on feedback control strategies only to regulate feed flow rates, boil up rates, and reflux rates (or ratios), at pre-determined settings that are updated only infrequently. While this is often termed automatic control, it is regarded as merely semiautomatic control in the context of this invention. When fixed column settings are periodically adjusted based on computerized steady state material balance calculations, using analysis of the feed, this method is sometimes described as computer process control, or control with computer simulation. However, the present invention considers these methods to be merely another type of semi-automatic control because they do not deal with the dynamic behavior of the process.
Semi-automatic control of a column that produces high purity products at both the distillate and bottoms is often effectively unstable, because very small disturbances in feed composition or flow rate will radically alter the purity of either bottoms or distillate. The present invention provides a method of stable, fully automatic control of controlling high purity distillation columns without any direct measurement of distillate or bottoms composition.
Semi-automatic control methods consume excess energy in order to incrementally increase the separation efficiency of a column above the minimum required. This margin allows fluctuations in product purity to be acceptable when the products are blended in storage or downstream operations. By employing the fully automatic control methods of this invention, a reduction in energy usage of 1% to 30% may be achieved by eliminating the need for this marginal excess. In addition, because the production rate of a column is limited by the design of the column to efficiently mix and separate vapor and liquids, a reduction in energy usage per unit of feed also allows a simultaneous increase in the production capacity of a column. Finally, greater consistency of product compositions by improved disturbance rejection can have a beneficial impact on the performance of downstream reactions and separations.
Prior art for distillation control also includes techniques of variable pairing and relative gain analysis to deal with the fact that the controls on a distillation column have affects on both overhead and bottoms compositions. For example, relative gain analysis attempts to use the steady state affects of small disturbances in boil-up rate and reflux rate on both overhead and bottoms composition to improve control response. These methods do not work as well as might be expected because the time-dependencies of each control are different for distillate than for bottoms compositions, and because they are non-linear. Detailed dynamic simulation also reveals that the apparent time constant for the response to a change in a manipulated variable is strongly dependent on the magnitude of the change. This invention effectively deals with non-linear behavior in the time domain.
To summarize, every existing approach to distillation control makes some false assumptions about distillation dynamics. Traditional PID control works best on systems where each manipulated variable has a predominant effect on only one property, and in a linear, stationary way. Because none of these assumptions are correct, PID control can stabilize column operation only in narrow operating regions, and often fails to solve any of the problems mentioned at the beginning of this paper. Furthermore, PID control optimally tuned for any one set of conditions may be unstable following a set point change.
Dynamic matrix control uses a stationary linear model of interaction between column variables, so while this approach often provides better control than PID, it is also limited to narrow operating range. Dynamic matrix models require time and expertise to build, and yet for distillation, these models will be invalidated any time the column operating conditions change significantly.
Non-linear model predictive control approaches allow for non-linear interaction between column variables, and may improve on dynamic matrix control in some cases, but are much more computationally complex, and the models are more problematic to create. Neural network based control is potentially easier to implement because the non-linear neural networks model can be fit to dynamic data from a column rather than requiring expert knowledge. However, the optimum structure of a neural network for distillation is unknown, and probably different for each application. Also, the neural network model is purely empirical and does not conform to mass or energy balances. Stable performance of a neural network control cannot be guaranteed, especially for operation outside of the training set conditions.
Sophisticated decoupling strategies for PID control can be implemented based on steady-state simulation updated with current column conditions. Decoupled controls may be properly referred to as simulation or non-linear model-based control, but because the simulation is based on steady-state models, the short term behavior of the process will not match that of the steady-state simulation, and control performance suffers may of the same problems as before.